1. Field of the Invention
This invention relates to chemical vapor deposition of thin films. Particularly, this invention relates to an apparatus and method for liquid injection and vaporization within a chemical vapor deposition process.
2. Description of Related Art
In a chemical vapor deposition (CVD) process, species in the vapor phase are injected over a substrate (or wafer) and react to form a deposit on the substrate. Often, this is done with plasma enhancement (PECVD). This technique provides for superior coverage of complex topographies on the substrate surface. The introduction of deposition reactants and other dopants has posed technical problems in the art that are governed, in part, by the physical properties these materials undergo within the chemical vapor deposition process. Some resolutions to these problems have incorporated direct injection of liquid solutions to introduce a deposition reactant into a CVD deposition chamber. Other answers have considered vaporizing pre-mixed solutions upon injection into the chamber. Typically, a solution is pre-mixed and produced outside the deposition chamber and transported to the chamber under temperature and pressure conditions appropriate for maintaining the solution in a liquid state until injection. Once injected, the solution then vaporizes for efficient mixing within the chamber and subsequent deposition. For the purposes of this disclosure, the term “solution” refers to any multi-component homogeneous single-phase liquid, regardless of the relative concentrations or states of aggregation, complexing, or bonding of the components in the liquid. The injected solution is applied within the chamber typically by an injection nozzle, where, under proper temperature and pressure conditions, the solution is allowed to enter the vapor phase over the substrate, react, and deposit a film.
In U.S. Pat. No. 5,527,567 issued to Desu, et al., on 18 Jun. 1996, entitled, “METALORGANIC CHEMICAL VAPOR DEPOSITION OF LAYERED STRUCTURE OXIDES,” a chemical vapor deposition process is taught in which the vapors are introduced into the reactor either through a set of bubblers or through a direct liquid injection system. A flash vaporization chamber (basically a pump with a liquid flow meter) is employed along with a needle valve inserted within the flow line to control the flow of liquid. A preheated carrier gas is used to transport the pre-mixed vaporized source from the vaporization chamber.
In U.S. Pat. No. 5,968,594 issued to Hu et al., on 19 Oct. 1999, entitled “DIRECT LIQUID INJECTION OF LIQUID AMMONIA SOLUTIONS IN CHEMICAL VAPOR DEPOSITION,” reactants are delivered by direct liquid injection. The reactant solution is produced outside the deposition chamber and transported to the chamber under temperature and pressure conditions appropriate for maintaining the solution in a liquid state until injection. The liquid solution serves as a vehicle to transport reactant material to the deposition chamber.
The present methods in the art used to inject a carrier fluid (gas) such as tetraethylorthosilicate (TEOS) along with precursors and dopants into the gas manifolding leading to the reactor chamber is inefficient and does not fully vaporize the injected fluids, especially the TEOS. In addition, the heat supplied to the gas manifold necessary to heat the carrier fluid is optimized to heat a liquid which has been fully atomized. When using TEOS as a carrier fluid, if the fluids do not atomize upon entering the gas manifold, the heat transfer of the fluids is then limited by the large droplets of TEOS on the manifold structure. The result is a mixing bowl which fills with TEOS liquid and inefficient deposition of silicon oxide on the process wafers with the potential risk of carrier gas material landing on the wafer. Although TEOS is described as an illustrative example, other fluids commonly used in the art will have similar constraints.
In addition to the inefficient atomization of the liquid(s) entering the process chamber, flow rate control presents another concern. The flow rates required for TEOS are typically in the range of 0-6 ml/min. The flow rates for the dopants used in the TEOS processes are in the range of 0-1 ml/min. Presently, the methods used to introduce these low flow rates of dopants and TEOS into the chamber require pre-mixing of the liquids. Once these liquids are pre-mixed, they are passed through a high pressure drop capillary tube which drops the pressure of the liquids to the low pressure levels required for vaporization and introduction within the process chamber. This high pressure drop capillary tube also prevents cavitation from occurring in the liquid supply lines. One difficulty with using capillary tubes, however, is the inherent limitations on pressure drops. The pressure drop in a capillary tube is inversely proportional to its length, and proportional to the fourth power of the diameter of the tube (D4/L). As a result, depending upon the flow rate required, either the pressure drop across the capillary tube will be excessive or the capillary tube will need to be extremely long to meet the flow rate required for the specific process. Moreover, capillary injector tubes are generally sophisticated in their configuration, and difficult to manufacture and service.
As a result of pre-mixing liquids prior to their introduction into the chamber, minor pressure fluctuations in any of the multiple liquid flow supplies will cause the dopant flow rate to fluctuate. Since the dopant flow levels are much lower than the TEOS flow rates, minor changes in the TEOS flow will cause large changes in the dopant flow. This phenomena is sometimes referred to as “crosstalk.” Additionally, high pressures are typically required to drive the liquids through the restrictive capillary tubes. As a result, a delay of tens of seconds is required in order for the dopant to stabilize. Furthermore, the explosive nature of certain fluids, such as TEOS, requires the introduction of these fluids under safe temperature and pressure conditions.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an apparatus and method to atomize and vaporize precursors and dopants in a chemical vapor deposition process more thoroughly and efficiently.
It is another object of the present invention to provide an apparatus and method which allows for the separate introduction of different liquids with widely different flow rates, including low flow rate dopant liquids, into a CVD process chamber at constant pressure.
A further object of the invention is to provide an apparatus and method for effective atomization and vaporization of any liquid precursor prior to its introduction into a reactive chamber.
It is yet another object of the present invention to provide an apparatus and method to introduce liquids into a chemical vapor deposition chamber in a constant pressure environment without pre-mixing the liquids.
Another object of the present invention is to provide an apparatus and method which prevents TEOS or other liquids which are explosive in the presence of oxygen at high pressure and temperature from reaching their auto-ignition point.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.